Semiconductor device with alternating conductivity type layer and method of manufacturing the same

ABSTRACT

A semiconductor device having an alternating conductivity type layer improves the tradeoff between the on-resistance and the breakdown voltage and facilitates increasing the current capacity by reducing the on- resistance while maintaining a high breakdown voltage. The semiconductor device includes a semiconductive substrate region, through which a current flows in the ON-state of the device and that is depleted in the OFF-state. The semiconductive substrate region includes a plurality of vertical alignments of n-type buried regions  32  and a plurality of vertical alignments of p-type buried regions. The vertically aligned n-type buried regions and the vertically aligned p-type buried regions are alternately arranged horizontally. The n-type buried regions and p-type buried regions are formed by diffusing respective impurities into highly resistive n-type layers  32   a  laminated one by one epitaxially.

FIELD OF THE INVENTION

The present invention relates to a vertical semiconductor structure thatfacilitates realizing both a high breakdown voltage and a high currentcapacity in insulated gate field effect transistors (MOSFETs), insulatedgate bipolar transistors (IGBTs), bipolar transistors, diodes and suchsemiconductor devices. The present invention also relates to a method ofmanufacturing the semiconductor device with such a verticalsemiconductor structure.

BACKGROUND OF THE INVENTION

Semiconductor devices may be roughly classified as lateral semiconductordevices wherein electrodes are arranged on a major surface and verticalsemiconductor devices wherein electrodes are distributed on both majorsurfaces opposing each other. When the vertical semiconductor device isON, a drift current flows in the expansion direction of a drift layer,which becomes depleted by the reverse bias voltage when the verticalsemiconductor device is OFF. FIG. 19 is a cross section of aconventional planar n-channel vertical MOSFET. Referring now to FIG. 19,this vertical MOSFET includes a drain electrode 18; an n+-type drainlayer 11 with low resistance, with which drain electrode 18 is inelectrical contact; a highly resistive n⁻-type drift layer 12 on n+-typedrain layer 11; a p-type base region 13 a selectively formed in thesurface portion of n⁻-type drift layer 12; a heavily doped n⁺-typesource region 14 selectively formed in p-type base region 13 a; a gateelectrode layer 16 above the extended portion of p-type base region 13 aextended between n+-type source region 14 and n⁻-type drift layer 12; agate oxide film 15 between gate electrode layer 16 and the extendedportion of p-type base region 13 a; a source electrode 17 in commoncontact with the surfaces of n⁺-type source region 14 and p-type baseregion 13 a; and a drain electrode 18 on the back surface of n⁺-typedrain layer 11.

In the vertical semiconductor device as shown in FIG. 19, highlyresistive n⁻-type drift layer 12 works as a region for making a driftcurrent flow vertically when the MOSFET is in the ON-state. Highlyresistive n⁻-type drift layer 12 is depleted when the MOSFET is in theOFF-state, resulting in a high breakdown voltage of the MOSFET.Shortening the current path in highly resistive n⁻-type drift layer 12is effective for substantially reducing the on-resistance (resistancebetween the drain and the source) of the MOSFET, since the driftresistance is lowered. However, the short current path in n⁻-type driftlayer 12 causes breakdown at a low voltage, since the expansion width ofthe depletion layer that expands from the pn-junction between p-typebase region 13 a and n⁻-type drift layer 12 is narrowed and the electricfield strength soon reaches the maximum (critical) value for silicon. Ina semiconductor device with a high breakdown voltage, thecharacteristically thick n⁻-type drift layer 12 causes highon-resistance and therefore, losses increase. In short, there exists atradeoff between the on-resistance and the breakdown voltage of theMOSFET. This tradeoff between the on-resistance and the breakdownvoltage also exists in other semiconductor devices such as IGBTs,bipolar transistors and diodes. The tradeoff between the on-resistanceand the breakdown voltage is also present in lateral semiconductordevices, in which the flow direction of the drift current in theON-state of the devices is different from the expansion direction of thedepletion layer in the OFF-state of the device.

EP0053854, U.S. Pat. No. 5,216,275, U.S. Pat. No. 5,438,215 and JapaneseUnexamined Laid Open Patent Application H09 (1997)-266311 disclosesemiconductor devices that include a drift layer including heavily dopedn-type regions and p-type regions alternately laminated with each otherto solve the foregoing problems. The alternately laminated n-typeregions and p-type regions are depleted to bear the breakdown voltage inthe OFF-state of the device.

FIG. 20 is a cross section of a part of the vertical MOSFET according toan embodiment of U.S. Pat. No. 5,216,275. The vertical MOSFET of FIG. 20is different from the vertical MOSFET of FIG. 19 in that the verticalMOSFET of FIG. 20 includes a drift layer 22, that is not single-layered,but consists of n-type drift regions 22 a and p-type drift regions 22 balternately laminated with each other. In the figure, there is a p-typebase region 23 a, an n+-type source region 24, a gate electrode 26, asource electrode 27, and a drain electrode 28.

Drift layer 22 is formed in the following manner. First, a highlyresistive n-type layer is grown epitaxially on an n⁺-type drain layer21. The n⁻-type drift regions 22 a are formed by etching the highlyresistive n-type layer to form trenches down to n⁺-type drain layer 21.Then, p-type drift regions 22 b are formed by epitaxially growing p-typelayers in the trenches.

Hereinafter, the semiconductor device, including an alternatingconductivity type drift layer that makes a current flow in the ON-stateof the device and is depleted in the OFF-state of the device, will bereferred to as a “semiconductor device with an alternating conductivitytype layer.”

The dimensions described in U.S. Pat. No. 5,216,275 are as follows. Whenthe breakdown voltage is put in VB, the thickness of the drift layer 22is 0.024 V_(B) ^(1.2) (μm). When n-type drift region 22 a and p-typedrift region 22 b have the same thickness b and the same impurityconcentration, the impurity concentration is 7.2×10¹⁶ V_(B) ^(−0.2)/b(cm⁻³). If V_(B) is 800 V and b μm, the drift layer 22 will be 73 μm inthickness and the impurity concentration 1.9×10¹⁶ cm⁻³. Since theimpurity concentration for the single-layered drift layer is around2×10¹⁴ cm⁻³, the on-resistance is reduced. However, when usingconventional epitaxial growth techniques, it is difficult to bury a goodquality semiconductor layer in such a narrow and deep trench (with alarge aspect ratio).

The tradeoff between the on-resistance and the breakdown voltage is alsocommonly encountered in lateral semiconductive devices. The foregoingreferences, EP0053854, U.S. Pat. No. 5,438,215 and Japanese UnexaminedLaid Open Patent Application H09(1997)-266311, disclose lateralsemiconductor devices with an alternating conductivity type layer andmethods, common to the lateral semiconductor devices and verticalsemiconductor devices, for forming the alternating conductivity typelayer which employ selective etching technique for digging trenches andepitaxial growth techniques for filling the trenches. In manufacturingthe lateral semiconductor device, it is relatively easy to employselective etching techniques and epitaxial growth techniques to form analternating conductivity type layer, since thin epitaxial layers arelaminated one by one.

However, it is difficult to employ the selective etching technique fordigging trenches and using an epitaxial growth technique for filling thetrenches in manufacturing the vertical semiconductor devices withalternating conductivity type layer as explained with reference to U.S.Pat. No. 5,216,275. Japanese Unexamined Laid Open Patent Application H09(1997)-266311 describes the nuclear transformation by a neutron beam andsuch radioactive beams. However, such nuclear transformation processesrequire large facilities and cannot be used easily.

OBJECTS AND SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the invention to provide asemiconductor device with alternating conductivity type layer thatreduces the tradeoff relation between the on-resistance and thebreakdown voltage.

It is another object of the invention to provide a semiconductor devicewith an alternating conductivity type layer and with a high breakdownvoltage that facilitates increasing the current capacity by reducing theon-resistance. It is still another object of the invention to provide amethod for manufacturing such a semiconductor device with alternatingconductivity type layer easily and with excellent mass-productivity.

According to an aspect of the invention, there is provided asemiconductor device including: a layer with low electrical resistance;a semiconductive substrate region having a first surface contacting thelayer with low electrical resistance and a second surface; one or moreelectrodes on the second surface of the semiconductive substrate region;and an electrode on the surface of the layer with low electricalresistance not contacting the semiconductive substrate layer; thesemiconductive substrate region providing a current path when thesemiconductor device is ON and being depleted when the semiconductordevice is OFF; the semiconductive substrate region including regions ofa first conductivity type and regions of a second conductivity type; theregions of the first conductivity type and the regions of the secondconductivity type being extended substantially in parallel to each othervertically and arranged alternately with each other horizontally; eachof the regions of the first conductivity type including a plurality ofsecond buried regions of the first conductivity type aligned verticallyat a predetermined pitch; each of the regions of the second conductivitytype including a plurality of first buried regions of the secondconductivity type aligned vertically at the predetermined pitch.

Advantageously, the first buried regions and the second buried regionsare located at almost the same depths from the surface of thesemiconductive substrate region.

Advantageously, the second buried regions are located near the midpointsbetween the depths, at which the first buried regions are located, fromthe surface of the semiconductive substrate region.

Since the above described semiconductive substrate region is depleted inthe OFF-state of the semiconductor device, the impurity concentrationsin the first buried regions or the second buried regions can beincreased. Thus, the on-resistance is lowered. Advantageously, thespacing I1 between the centers of the adjacent first buried regionsaligned vertically is from 2 to 10 μm. When the spacing I₁ exceeds 10μm, heat treatment should be conducted for an extended period of time tomake the first buried regions or the depletion layers which expand fromthe first buried regions continue to each other. When the spacing I₁ isless than 2 μm, growth of the highly resistive layer and impurity dopingby ion implantation should be repeated several times, resulting inincreased manufacturing steps which are generally undesirable formass-production.

Advantageously, a relational expression 0.5 d≦I₁≦2 d holds for thespacing I₁ between the centers of the adjacent first buried regionsaligned vertically and the average spacing 2 d between the centers ofthe horizontally adjacent first buried regions.

If one assumes that the impurities diffuse evenly in all directions, theupper buried regions and the lower buried regions continue to each otherand, at the same time, the first buried regions and the second buriedregions continue to each other, then I₁ should approximately equal d. IfI₁ differs significantly from d, heat treatment should be conducted foran extended period of time to make the upper buried regions and thelower buried regions continue to each other after the first buriedregions and the second buried regions have continued to each other or tomake the first buried regions and the second buried regions continueafter the upper buried regions and the lower buried regions havecontinued to each other. Thus, I₁ being significantly different from dis not desirable from the view point of efficient manufacturing.Therefore, the desirable range for I₁ is between 0.5 d and 2 d.

Advantageously, a relational expression I₀<I₁ holds for the spacing I₀between the upper surface of the layer with low electrical resistanceand the center of the lowermost first buried region and the spacing I₁between the centers of the adjacent first buried regions alignedvertically.

If I₀ is close to I₁, the highly resistive region remains with abouthalf the original thickness left. The remaining highly resistive regioncauses increased on-resistance. Therefore, it is preferable for I₀ to bemuch smaller than I₁.

Advantageously, the first buried regions aligned vertically continue toeach other.

Since the first buried regions of the second conductivity type aredisposed to expand depletion layers into the second buried regions ofthe first conductivity type, the vertically aligned first buried regionscan be separated as long as the spaces between them are narrow enough tomake the depletion layers continuous. However, the first buried regionssurely work as intended when they continue to each other.

Advantageously, the second buried regions aligned vertically continue toeach other.

Since the vertically aligned second buried regions provide a driftcurrent path, the highly resistive layer between them results inincreased on-resistance. Therefore, it is desirable for the verticallyaligned second buried regions to continue to each other. Since thesecond buried regions of the first conductivity type are disposed toexpand the depletion layers into the first buried regions of the secondconductivity type;, the vertically aligned second buried regions can beseparated as long as the spaces between them are narrow enough to makethe depletion layers continuous. The second buried regions also work asintended when they continue to each other.

Advantageously, the first buried regions and the second buried regionsare formed with stripes extending horizontally. Advantageously, thefirst buried regions are formed with a lattice or a honeycomb extendinghorizontally, and the second buried regions are in the horizontallylattice-shaped first buried regions or in the bores of the horizontallyhoneycomb-shaped first buried regions. Alternatively, the second buriedregions are formed with a lattice or a honeycomb extending horizontally,and the first buried regions are in the horizontally lattice-shapedsecond buried regions or in the bores of the horizontallyhoneycomb-shaped second buried region. Advantageously, the first buriedregions are distributed on the lattice points of a rectangular lattice,a triangular lattice or a hexagonal lattice, and the second buriedregion is between the horizontally adjacent first buried regions.Alternatively, the first buried regions are distributed on the latticepoints of a rectangular lattice, a triangular lattice or a hexagonallattice, and the second buried region is in the center of the unitlattice of the rectangular lattice, the triangular lattice or thehexagonal lattice.

Any patterns and configurations are acceptable so long as the selectedpattern facilitates expanding the depletion layers into the first buriedregions and the second buried regions. Advantageously, the averagespacing 2 d between the centers of the horizontally adjacent firstburied regions is from 2 to 20 μm.

When an impurity is diffused for about 0.3 μm from a window opened inthe surface of the epitaxial layer of about 0.4 μm in width, which isthe limit of the conventional lithographic techniques, 2 d is about 2μm. When 2 d exceeds 20 μm, the impurity concentrations should be around2×10¹⁵ cm⁻³ to deplete the first buried regions and the second buriedregions by applying a voltage of around 300V. When the impurityconcentration is about 2×10¹⁵ cm⁻³, it is not effective at reducing theon-resistance.

According to another aspect of the invention, the first buried regionsand the second buried regions are formed by diffusing respectiveimpurities into a highly resistive layer laminate epitaxially grown onthe layer with low electrical resistance. By the above describedmanufacturing method, the semiconductor device with an alternatingconductivity type layer is easily manufactured without requiringtrenches with a high aspect ratio and filling the trenches with buriedregions. In the semiconductor device manufactured by the methodaccording to the invention, impurity concentration distributions arecaused in the first buried regions and the second buried regions by theimpurity diffusion from limited impurity sources.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a first embodiment of theinvention.

FIG. 2(a) is a cross section along A-A of the semiconductor device withan alternating conductivity type layer of FIG. 1.

FIG. 2(b) is a cross section along B-B of the semiconductor device withalternating conductivity type layer of FIG. 1.

FIG. 3(a) is an impurity distribution profile along A-A of FIG. 1.

FIG. 3(b) is an impurity distribution profile along C-C of FIG. 1.

FIG. 3(c) is an impurity distribution profile along D-D of FIG. 1

FIGS. 4(a) through 4(d) are cross sections describing the steps formanufacturing the MOSFET with an alternating conductivity type layeraccording to the first embodiment of the invention.

FIGS. 5(a) and 5(b) are further cross sections describing the furthersteps for manufacturing the MOSFET with an alternating conductivity typelayer according to the first embodiment of the invention.

FIG. 6 is a cross section of a modification of the MOSFET with analternating conductivity type layer according to the first embodiment ofthe invention.

FIG. 7 is a top plan view showing an example of a planar arrangement ofthe first buried region and the second buried region.

FIG. 8 is a top plan view showing another example of a planararrangement of the first buried region and the second buried region.

FIG. 9 is a top plan view showing still another example of planararrangement of the first buried region and the second buried region.

FIG. 10 is a top plan view showing a further example of planararrangement of the first buried region and the second buried region.

FIG. 11 is a top plan view showing a still further example of a planararrangement of the first buried region and the second buried region.

FIG. 12 is a top plan view showing the other example of planararrangement of the first buried region and the second buried region.

FIG. 13 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a second embodiment of theinvention.

FIG. 14 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a third embodiment of theinvention.

FIG. 15 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a fourth embodiment of theinvention.

FIG. 16 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a fifth embodiment of theinvention.

FIG. 17 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a sixth embodiment of theinvention.

FIG. 18 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a seventh embodiment of theinvention.

FIG. 19 is a cross section of a conventional planar n-channel verticalMOSFET.

FIG. 20 is a cross section of a part of the vertical MOSFET according toan embodiment of U.S. Pat. No. 5,216,275.

Now the present invention will be described hereinafter with referenceto the accompanying drawing figures which illustrate the preferredembodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a first embodiment of theinvention. FIG. 2(a) is a cross section along A-A of the semiconductordevice with an alternating conductivity type layer of FIG. 1. FIG. 2(b)is a cross section along B-B of the semiconductor device with analternating conductivity type layer of FIG. 1.

Referring now to FIG. 1, a semiconductive substrate region 32 is formedon an n⁺-type, low resistance, drain layer 31. Semiconductive substrateregion 32 includes a highly resistive n⁻-type layer 32 a, a plurality ofvertical alignments of n-type buried regions 32 b and a plurality ofvertical alignments of p-type buried regions 32 c. The verticalalignments of n-type buried regions 32 b and the vertical alignments ofp-type buried regions 32 c are alternately arranged with each otherhorizontally. An n-type channel layer 32 d is in contact with theuppermost n-type buried region 32 b. A p-type base region 33 a is incontact with the uppermost p-type buried region 32 c. An n+-type sourceregion 34 and a heavily doped p+-type well region 33 b are in p-typebase region 33 a. A gate electrode layer 36 is above the extendedportion of p-type base region 33 a extended between n+-type sourceregion 34 and n-type channel layer 32 d with a gate insulation film 35interposed therebetween. A source electrode 37 is in contact with bothn+-type source region 34 and p+-type well region 33 b. A drain electrode38 is on the back surface of n+-type drain layer 31. Source electrode 37can be extended over gate electrode layer 36 with an insulation film 39interposed therebetween as shown in FIG. 1. Although the drift currentflows through n-type buried regions 32 b and highly resistive n⁺-typelayers 32 a, the semiconductive substrate region 32 including p-typeburied regions 32 c, will be collectively referred to hereinafter as the“drift layer”.

Broken lines in FIG. 1 indicate the planes at which the formation of thesemiconductive substrate region 32 is interrupted and where impuritiesare implanted. The n-type buried regions 32 b and p-type buried regions32 c are formed by impurity diffusion from the respective impuritysources implanted into the central portions of n-type buried regions 32b and p-type buried regions 32 c. Although the pn-junctions betweenn-type buried regions 32 b and p-type buried regions 32 c arerepresented by curves in FIG. 1 (curved surfaces three-dimensionally)due to their formation technique described above, the pn-junctionsbetween n-type buried regions 32 b and p-type buried regions 32 c tendto gradually straighten (to be flat planes three-dimensionally) as theheat treatment period for diffusion is extended.

As illustrated in FIG. 2(a), n-type buried regions 32 b and p-typeburied regions 32 c extend horizontally as stripes. In FIG. 2(a), n-typeburied regions 32 b and p-type buried regions 32 c are in contact toeach other. In FIG. 2(b), i.e., along B-B of FIG. 1, n-type buriedregions 32 b and p-type buried regions 32 c are not in contact with eachother, the regions being separated with the highly resistive n⁻-typelayer 32 a left therebetween. The highly resistive n⁻-type layer 32 abetween n-type buried regions 32 b and p-type buried regions 32 c may benarrowed and ultimately fade away by extending the time period of theheat treatment subsequent to ion implantation.

FIG. 3(a) is an impurity distribution profile along A-A of FIG. 1. FIG.3(b) is an impurity distribution profile along C-C of FIG. 1. FIG. 3(c)is an impurity distribution profile along D-D of FIG. 1. In thesefigures, the vertical axis represents the logarithmic impurityconcentration. In FIG. 3(a), the impurity distributions caused by thediffusion from the respective impurity sources of n-type buried regions32 b and p-type buried regions 32 c arranged alternately are repeated.In FIG. 3(b), the impurity distributions caused by the diffusion fromthe diffusion sources of n-type buried regions 32 b are continuous andrepeated vertically above n+-type drain layer 31 with low resistance.Impurity distribution is also caused in n-type channel region 32 d onthe uppermost n-type buried region 32 b by the diffusion from thesurface of n-type channel region. In FIG. 3(c), the impuritydistributions caused by the diffusion from the diffusion sources ofp-type buried regions 32 c are continuous and repeated vertically aboven+-type drain layer 31 with low resistance. Impurity distributions arealso caused in p-type base region 33 a and p⁺-type well region 33 bcontinuous to the uppermost p-type buried region 32 c by the diffusionfrom their surfaces.

The semiconductor device with an alternating conductivity type layer ofFIG. 1 operates as follows. When a predetermined positive voltage isapplied to gate electrode layer 36, an inversion layer is caused in thesurface portion of p-type base region 33 a beneath gate electrode layer36. The electrons injected to p⁺-type channel region 33 b from n⁺-typesource region 34 via the inversion layer reach n⁺-type drain layer 31via n-type buried regions 32 b, resulting in electrical conductionbetween drain electrode 38 and source electrode 37.

As the positive voltage is removed from gate electrode layer 36, theinversion layer induced in the surface portion of p-type base region 33a vanishes, and the drain 38 and the source 37 are electricallydisconnected from each other. As the reverse bias voltage is furtherboosted, depletion layers expand into n-type buried regions 32 b andp-type buried regions 32 c from the pn-junctions Ja between p-type baseregions 33 a and n-type channel regions 32 d, the pn-junctions Jbbetween p-type buried regions 32 c and n-type buried regions 32 b andthe pn-junctions Jc between p-type buried regions 32 c and highlyresistive n⁻-type layer 32 a. Thus, n-type buried regions 32 b andp-type buried regions 32 c are depleted.

The n-type buried regions 32 b are depleted very quickly, since theedges of the depletion layers from the pn-junctions Jb and Jc advance inthe width direction of n-type buried regions 32 b and since depletionlayers also expand into n-type buried regions 32 b from p-type buriedregions 32 c on both sides of n-type buried regions 32 b. Therefore,n-type buried regions 32 b may be heavily doped.

The p-type buried regions 32 c are depleted simultaneously with n-typeburied regions 32 b. The p-type buried regions 32 c are depleted veryquickly, since depletion layers expand into p-type buried regions 32 cfrom both sides of the regions. Since the edges of the depletion layerfrom p-type buried region 32 c enter adjacent n-type buried regions 32 bby virtue of the alternating arrangement of p-type buried regions 32 cand n-type buried regions 32 b, the total width occupied by p-typeburied regions 32 c for forming depletion layers may be halved.Accordingly, the cross sectional area of n-type buried regions 32 b maybe widened.

The dimensions and the impurity concentrations for an exemplary MOSFETof the 300 V class are as follows. The thickness of n⁺-type drain layer31 is 0.01 μm. The thickness of n+-type drain layer 31 is 350 μm. Thespecific resistance of highly resistive n⁻-type drain layer 32 a is 10Ω·cm. The thickness of drift layer 32 is 25 μm (5 μm each for I₀, I₁, .. . ). The n-type buried region 32 b and p-type buried region 32 c are 5μm in width: the spacing between the centers of the buried regions withthe same conductivity type is 10 μm. The average impurity concentrationin the n-type buried regions 32 b and p-type buried regions 32 c is7×10¹⁵ cm⁻³. The diffusion depth of p-type base region 33 a is 1 μm. Thesurface impurity concentration of p-type base region 33 a is 3×10¹⁸cm⁻³. The diffusion depth of n+-type source region 34 is 0.3 μm. Thesurface impurity concentration of n⁺-type source region 34 is 1×10²⁰cm⁻³.

To provide a conventional vertical MOSFET with a breakdown voltage ofabout 300 V, it is necessary for a single-layered highly resistive driftlayer to contain an impurity concentration of around 2×10¹⁴ cm⁻³ and tobe about 40 μm in thickness. In the MOSFET with an alternatingconductivity type layer according to the first embodiment of theinvention, its on-resistance is reduced to one fifth of that of theconventional vertical MOSFET by increasing the impurity concentration inn-type buried regions 32 b and by reducing the thickness of the driftlayer 32 in accordance with the increment of the impurity concentration.

The on-resistance is further reduced and the tradeoff relation betweenthe on-resistance and the breakdown voltage is further improved byreducing the width of n-type buried regions 32 b and by furtherincreasing its impurity concentration.

The MOSFET with an alternating conductivity type layer according to thefirst embodiment of the invention results in a novel structure of driftlayer 32. Since the n-type buried regions 32 b and p-type buried regions32 c of drift layer 32 in the MOSFET according to the invention areformed by impurity diffusion, impurity distributions are caused in driftlayer 32 by such diffusion.

FIGS. 4(a) through 4(d) and FIGS. 5(a) and 5(b) are cross sectionsdescribing the steps for manufacturing the MOSFET with alternatingconductivity type layer according to the first embodiment of theinvention.

Referring now to FIG. 4(a), a highly resistive n⁻-type layer 32 a isgrown on an n⁺-type drain layer 31 that works as an n-type substratewith low resistance. In this embodiment, the thickness I₀ of n⁻-typelayer 32 a deposited at first is set at about 4 μm.

Referring now to FIG. 4(b), a photoresist mask pattern 1 is formed onthe n⁻-type layer 32 a and boron ions (hereinafter referred to as “Bions” 2 are implanted under the acceleration voltage of 50 keV and atthe dose amount of 1×10¹³ cm⁻². The implanted B ions are designated bythe reference numeral 3.

Referring now to FIG. 4(c), a photoresist mask pattern 2 is formed onthe n⁻-type layer 32 a and phosphorus ions (hereinafter referred to as“P ions” 5 are implanted under the acceleration voltage of 50 keV and atthe dose amount of 1×10¹³ cm⁻². The implanted P ions are designated bythe reference numeral 6.

Referring now to FIG. 4(d), an additional highly resistive n⁻-type layer32 a is grown to the thickness of I₁, photoresist mask patterns areformed, and B and P ions are implanted in a similar manners as describedabove with reference to FIGS. 4(a) through 4(c). These steps arerepeated until drift layer 32 has been grown to the predeterminedthickness. In this embodiment, the thickness I₁ of the additional highlyresistive n⁻-type layers is set at 5 μm and three additional n⁻-typelayers are laminated. And, a layer for forming a surface portion isgrown on the uppermost highly resistive n⁻-type layer.

Referring now to FIG. 5(a), n-type buried regions 32 b and p-type buriedregions 32 c are formed by diffusing the implanted impurities by heattreatment conducted at 1150° C. for 5 hr. The impurities diffuse in aregion of about 3 μm by this heat treatment, resulting in contact ofn-type buried regions 32 b and p-type buried regions 32 c to each other.The final shapes of n-type buried regions 32 b and p-type buried regions32 c may be varied by changing the shape of the masks for ionimplantation, the dose amounts of the impurities and the time period ofthe heat treatment.

Referring now to FIG. 5(b), n-type channel region 32 d, p-type baseregion 33 a, n⁺-type source region 34 and p⁺-type well region 33 b areformed in the surface portion in the same manner as those in theconventional vertical MOSFET by selective impurity ion implantation andby the subsequent heat treatment. Gate insulation film 35 is then formedby thermal oxidation. Gate electrode layer 36 is formed by depositingpolycrystalline silicon film by the vacuum CVD technique and by thesubsequent photolithographic process. Then, insulation film 39 isdeposited and windows are opened through insulation film 39. Sourceelectrode 37, drain electrode 38 and the not shown metallic portion ofthe gate electrode are formed by depositing an aluminum alloy layer andby patterning the deposited aluminum alloy layer. Thus, the verticalMOSFET as shown in FIG. 1 is completed.

While it is known to form buried regions by growing epitaxial layers ofseveral μm in thickness and by thermally diffusing implanted impurityions, the technique employed in the invention facilitates manufacturingthe MOSFET with an alternating conductivity type layer that improves thetradeoff between the on-resistance and the breakdown voltage without theneed to form trenches with a large aspect ratio and filling each trenchwith a high-quality epitaxial layer.

If highly resistive n⁻-type layer 32 a remaining below n-type buriedregion is thick, the on-resistance will be high. Therefore, thethickness I₀ of the epitaxial layer on n⁺-type drain layer 31 ispreferably set to be thinner than the thickness I₁ of the next epitaxiallayer. FIG. 6 is a cross section of a modification of the MOSFET with analternating conductivity type layer according to the first embodiment ofthe invention. This modified MOSFET is manufactured by extending thetime period for the heat treatment following the impurity ionimplantation and the epitaxial growth described with reference to FIG.4(d). Due to the extended heat treatment, highly resistive n⁻-type layer32 a has been eliminated and adjacent n-type buried regions 32 b andp-type buried regions 32 c are contacting each other over most of theirentire side faces, resulting in substantially flat boundaries. Theresulting flat boundaries between n-type buried regions 32 b and p-typeburied regions 32 c are illustrated by straight lines in FIG. 6.

The cross section shown in FIG. 6 for the modification resembles thecross section shown in FIG. 20 for the conventional MOSFET. However,their internal semiconductor structure are different from each other. InFIG. 20, the impurity concentration is almost uniform in each epitaxiallayer, since the epitaxial layers are formed by growing a firstepitaxial layer and by filling the trenches dug in the first epitaxiallayer with second epitaxial layers. In contrast, the impuritydistribution profiles along section lines E-E, F-F and G-G of FIG. 6 areessentially the same with those described in FIG. 3(a), FIG. 3(b), andFIG. 3(c), respectively. The impurity distribution due to highlyresistive n⁻-type layer 32 a does not appear in the profiles along E-E,F-F and G-G of FIG. 6, since any resistive n⁻-type layer 32 a is notremaining in FIG. 6. the impurity distribution profile along E-E of FIG.6 includes the impurity distributions across n-type buried regions 32 band p-type buried regions 32 c alternately arranged with each otherhorizontally. The impurity distribution profile along lines F-F of FIG.6 includes the impurity distribution across n+ drain layer 31, cyclicconcentration change due to the diffusion from the sources in n-typeburied regions 32 b and the impurity distribution across n-type channelregion 32 d in the surface portion. The impurity distribution profilealong G-G of FIG. 6 includes the impurity distribution across n+ drainlayer 31, cyclic concentration change due to the diffusion from thesources in p-type buried regions 32 c, the impurity distribution acrossp-type base region 32 a and the impurity distribution across p⁺-typewell region 33 b in the surface portion.

Although n-type buried regions 32 b and p-type buried regions 32 c arearranged in stripes horizontally in the first embodiment of theinvention, n-type buried regions 32 b and p-type buried regions 32 c maybe arranged in different fashions. FIGS. 7 through 12 show variousplanar arrangements of n-type buried regions 32 b and p-type buriedregions 32 c. In FIG. 7, a matrix of rectangular n-type buried regions32 b is arranged in p-type buried region 32 c. In FIG. 8, a matrix ofrectangular p-type buried regions 32 c is arranged in n-type buriedregion 32 b. Alternatively, n-type buried region 32 b or p-type buriedregion 32 c is shaped with a honeycomb, the bores of which are filledwith p-type buried regions 32 c or n-type buried regions 32 b.

FIGS. 9 through 12 show the examples of scattered arrangement. In FIG.9, p-type buried regions 32 c are arranged at the lattice points of asquare lattice and n-type buried region 32 b is arranged betweenadjacent p-type buried regions 32 c. In FIG. 10, p-type buried regions32 c are arranged at the lattice points of a square lattice and n-typeburied region 32 b is arranged in the center of each unit lattice. InFIG. 11, p-type buried regions 32 c are arranged at the lattice pointsof a triangular lattice and n-type buried region 32 b is arrangedbetween adjacent p-type buried regions 32 c. In FIG. 12, p-type buriedregions 32 c are arranged at the lattice points of a triangular latticeand n-type buried region 32 b is arranged in the center of the unitlattice. In these arrangements, the spacing between n-type buried region32 b and p-type buried region 32 c is narrowed by extending the timeperiod for heat treatment conducted following epitaxial growth andimpurity ion implantation. In some cases, a p-type layer may be used insubstitution for highly resistive n⁻-type layer 32 a. Other variousrepetitive arrangements may be adopted in addition to those disclosedwith particularity herein.

It is not always necessary to shape p-type base region 33 a in thesurface portion and p-type buried region 32 c with similar planarpatterns. The p-type base region 33 a and p-type buried region 32 c maybe shaped with quite different respective patterns so long as they areconnected to each other. For example, p-type base region 33 a and p-typeburied region 32 c may be shaped with respective stripe patterns, whichextend perpendicularly to each other.

In any arrangement, the contact area between n-type buried regions 32 band p-type buried regions 32 c is widened gradually as the time periodfor thermal diffusion is extended and highly resistive n⁻-type layer 32a is narrowed gradually until it finally vanishes.

In the first embodiment, heat treatment is employed for inducingdiffusion and connecting the upper and lower n-type buried regions 32 b.When the highly resistive layer is n⁻-type layer 32 a, it is not alwaysnecessary to connect the upper and lower n-type buried regions 32 b.However, n⁻-type layer 32 a remaining between n-type buried regions 32 bincreases the on-resistance. It is not always necessary to connect theupper and lower p-type buried regions 32 c so long as they are spacedapart with a distance short enough for the depletion layers to join eachother.

Second Embodiment

FIG. 13 is a cross section of a semiconductor device with alternatingconductivity type layer according to a second embodiment of theinvention.

Referring now to FIG. 13, a drift layer 42 is on an n⁺-type drain layer41 with low resistance. Drift layer 42 includes a highly resistive layer42 a, a plurality of vertical alignments of n-type buried regions 42 band a plurality of vertical alignments of p-type buried regions 42 c.The vertical alignments of n-type buried regions 42 b and the verticalalignments of p-type buried regions 42 c are alternately arranged witheach other horizontally. In the surface portion of drift layer 42,n-type channel layers 42 d are formed on the uppermost n-type buriedregions 42 b and p-type base regions 43 a on the uppermost p-type buriedregions 42 c. An n⁺-type source region 44 and a heavily doped p⁺-typewell region 43 b are in p-type base region 43 a. A gate electrode layer46 is above the extended portion of p-type base region 43 a extendedbetween n⁺-type source region 44 and n-type channel layer 42 d with agate insulation film 45 interposed therebetween. A source electrode 47is in contact with both the n⁺-type source region 44 and the heavilydoped p⁺-type well region 43 b. A drain electrode 48 is on the backsurface of n+-type drain layer 41.

The MOSFET with alternating conductivity type layer according to thesecond embodiment of the invention is different from the MOSFET with analternating conductivity type layer according to the first embodiment inthe way of forming its drift layer and the resulting structure of thedrift layer. In the second embodiment, the n-type buried regions 42 band the p-type buried regions 42 c are formed by the diffusion of therespective impurities implanted in the surface portions of differentepitaxial layers.

It is not always necessary to implant the impurities for the n-typeburied regions and the p-type buried regions at the same depth, as inthe first embodiment. The n-type buried regions 42 b and p-type buriedregions 42 c may be at different depths.

The MOSFET with an alternating conductivity type layer according to thesecond embodiment of the invention exhibits the same effects as those ofthe MOSFET with an alternating conductivity type layer according to thefirst embodiment. The MOSFET with an alternating conductivity type layerthat improves the tradeoff relation between the on-resistance and thebreakdown voltage according to the second embodiment is manufactured byepitaxial growth and impurity diffusion techniques without the need toform trenches with a large aspect ratio and filling each trench with ahigh-quality epitaxial layer.

The n-type buried regions 42 b and the p-type buried regions 42 c may bearranged two dimensionally in a stripe pattern, in a lattice pattern, orin a scattered fashion in the second and following embodiments in thesame manner as in the first embodiment. In the second embodiment, it isnot always necessary for the highly resistive layer 42 a to be of n-typeand a highly resistive p-type layer is also acceptable, since n-typeburied regions 42 b are in contact with n+-type drain layer 41. When thehighly resistive layer 42 a is of a p-type, it is not necessary for theupper and lower p-type buried regions 42 c to be in contact with eachother.

Third Embodiment

FIG. 14 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a third embodiment of theinvention.

The semiconductor device shown in FIG. 14 is a UMOSFET that includestrench gates. The gate structure of the UMOSFET is different from thatof the MOSFET according to the first embodiment. Referring now to FIG.14, trenches are dug in the surface portion of a drift layer 52. A gateelectrode layer 56 is surrounded by a gate insulation film 55 in thetrench. In the remaining surface portion of drift layer 52, p-type baselayers 53 a are formed in the depth as shallow as that of gate electrodelayer 56, and n+ source regions 54 are formed along the upper edges ofgate electrode layers 56. A thick insulation film 59 covers gateelectrode layers 56. Drift layer 52 includes a plurality of verticalalignments of n-type buried regions 52 b and a plurality of verticalalignments of p-type buried regions 52 c, in a similar manner as in theforegoing embodiments. The vertical alignments of n-type buried regions52 b and the vertical alignments of p-type buried regions 52 c arealternately arranged with each other horizontally.

In the third embodiment, n-type buried region 52 b and p-type buriedregion 52 c have dimensions and impurity concentrations which areapproximately the same as those described in connection with the firstembodiment. When a reverse bias voltage is applied, drift layer 52 isdepleted to bear the breakdown voltage.

Since n-type buried regions 52 b and p-type buried regions 52 c areeasily depleted, they can be doped heavily. The drift layer 52 can bethinned by virtue of the heavy doping to n-type buried regions 52 b andp-type buried regions 52 c, thus the on-resistance is reduced and thetradeoff relation between the on-resistance and the breakdown voltage isimproved. By using techniques such as epitaxial growth and impuritydiffusion, the UMOSFET with an alternating conductivity type layer thatimproves the tradeoff relation between the on-resistance and thebreakdown voltage is easily manufactured.

Fourth Embodiment

FIG. 15 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a fourth embodiment of theinvention.

The semiconductor device shown in FIG. 15 is an n-channel IGBT. Thedrain layer structure of the IGBT is different from that of the MOSFETaccording to the first embodiment. In detail, this n-channel IGBT isobtained by adopting a binary- layer structure consisting of a p⁺-typedrain layer 61 a and an n+-type buffer layer 61 b in substitution forsingle-layered n⁺-type drain layer 21 of the MOSFET with alternatingconductivity type layer. In some cases, n⁺-type buffer layer 61 b may beomitted. The IGBT of FIG. 15 includes a drift layer 62 including aplurality of vertical alignments of n-type buried regions 62 b and aplurality of vertical alignments of p-type buried regions 62 c, similarto that described in the foregoing embodiments. The vertical alignmentsof n-type buried regions 62 b and the vertical alignments of p-typeburied regions 62 c are alternately arranged with respect to each otherhorizontally.

Since the IGBT is a semiconductor device of conductivity modulation typebased on minority carrier injection, its on-resistance is much smallerthan that of the MOSFET based on the drift of majority carriers. Evenso, the on-resistance of the IGBT is further reduced by thinning driftlayer 62.

A p-type substrate with low resistance is used for p⁺-type drain layer61 a. An epitaxial layer for n⁺-type buffer layer 61 b is grown on thep-type substrate with low resistance and, then, drift layer 62 is formedon n⁺-type buffer layer 61 b by epitaxial growth and impurity diffusion.

The techniques employed in the fourth embodiment facilitatemanufacturing the IGBT with alternating conductivity type layer thatimproves the tradeoff relation between the on-resistance and thebreakdown voltage without requiring trenches with a large aspect ratioand then filling each trench with a high-quality epitaxial layer.

Fifth Embodiment

FIG. 16 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a fifth embodiment of theinvention.

The semiconductor device of FIG. 16 is a diode that includes an n⁺-typecathode layer 71 with low resistance and a drift layer 72. Drift layer72 includes a highly resistive n⁻-type layer 72 a, a plurality ofvertical alignments of n-type buried regions 72 b and a plurality ofvertical alignments of p-type buried regions 72 c. The verticalalignments of n-type buried regions 72 b and the vertical alignments ofp-type buried regions 72 c are alternately arranged with each otherhorizontally. A p⁺-type anode layer 73 is on drift layer 72. An anodeelectrode 78 is in contact with p⁺-type anode layer 73. A cathodeelectrode 77 is in contact with n⁺-type cathode layer 71.

In the fifth embodiment, n-type buried regions 72 b and p-type buriedregions 72 c have dimensions and impurity concentrations about the sameas those set forth in the first embodiment. When a reverse bias voltageis applied, drift layer 72 is depleted to bear the breakdown voltage.

Since n-type buried regions 72 b and p-type buried regions 72 c areeasily depleted, they can be doped heavily. Since drift layer 72 can bethinned by virtue of the heavy doping to n-type buried regions 52 b andp-type buried regions 52 c, the on-resistance is reduced and thetradeoff relation between the on-resistance and the breakdown voltage isimproved.

Steps similar to those described with reference to FIGS. 4(a) through4(d) are employed for manufacturing the diode of FIG. 16. Then, p⁺-typeanode layer 73 is formed by ion implantation and subsequent diffusion.Finally, anode electrode 78 and cathode electrode 77 are formed.

Thus, a diode which exhibits a high breakdown voltage and a lowon-resistance is manufactured easily by quite general epitaxial growthand subsequent impurity diffusion.

Sixth Embodiment

FIG. 17 is a cross section of a semiconductor device with an alternatingconductivity type layer according to a sixth embodiment of theinvention.

The semiconductor device of FIG. 17 is another diode embodiment, whichincludes a drift layer 82, that is different from drift layer 72 of thefifth embodiment. Although drift layer 82 includes a highly resistiven⁻-type layer 82 a, a plurality of vertical alignments of n-type buriedregions 82 b and a plurality of vertical alignments of p-type buriedregions 82 c, the lowermost plane and the uppermost plane, in which theimpurity concentrations are highest, contact with an n⁺-type cathodelayer 81 and a p⁺-type anode layer 83, respectively. In the sixthembodiment, n-type buried regions 82 b and p-type buried regions 82 chave the dimensions and impurity concentrations about the same as thosedescribed in the first embodiments. When a reverse bias voltage isapplied, drift layer 82 is depleted to bear the breakdown voltage.

The diode of FIG. 17 reduces its on-resistance and improves the tradeoffrelation between the breakdown voltage and the on-resistance in the sameway as the diode of FIG. 16 does.

The diode which exhibits a high breakdown voltage and a lowon-resistance is manufactured easily in the same way as the diode ofFIG. 16.

Seventh Embodiment

FIG. 18 is a cross section of a semiconductor device with alternatingconductivity type layer according to a seventh embodiment of theinvention.

The semiconductor device of FIG. 18 is a Schottky diode that includes ann⁺-type cathode layer 91 and a drift layer 92. Drift layer 92 includes ahighly resistive n⁻-type layer 92 a, a plurality of vertical alignmentsof n-type buried regions 92 b and a plurality of vertical alignments ofp-type buried regions 92 c. The vertical alignments of n-type buriedregions 92 b and the vertical alignments of p-type buried regions 92 care alternately arranged with each other horizontally. Some parts ofhighly resistive n⁻-type layers 92 a are remaining in the surfaceportion of drift layer 92, and the uppermost p-type buried regions 92 care extended to the surface of drift layer 92. A Schottky electrode 98is on drift layer 92 such that Schottky barrier is formed between theSchottky electrode 98 and the remaining portion of highly resistiven⁻-type layer 92 a remaining in the surface portion of drift layer 92. Acathode electrode 97 is on the back surface of n⁺-type cathode layer 91.

In the Schottky diode with an alternating conductivity type layeraccording to the seventh embodiment, n-type buried regions 92 b andp-type buried regions 92 c have the dimensions and impurityconcentrations almost same as those in the first embodiments. When areverse bias voltage is applied, drift layer 92 is depleted to bear thebreakdown voltage. Since n-type buried regions 92 b and p-type buriedregions 92 c are easily depleted, they can be doped heavily. Since thethickness of the drift layer 92 can be reduced by virtue of the heavydoping to n-type buried regions 92 b and p-type buried regions 92 c, theon-resistance is reduced and the tradeoff relation between theon-resistance and the breakdown voltage is improved.

Steps similar to those described with reference to FIGS. 4(a) through4(d) are employed for manufacturing the Schottky diode of FIG. 18.Finally, Schottky electrode 98 and cathode electrode 97 are formed.

Thus, a Schottky diode that exhibits a high breakdown voltage and a lowon-resistance is easily manufactured using general epitaxial growth andsubsequent impurity diffusion techniques.

The semiconductor structure with an alternating conductivity type layeraccording to the invention is applicable to almost all the semiconductordevices such as the MOSFET, IGBT, diode, bipolar transistor, JFET,thyristor, MESFET, and HEMT. The conductivity types may be exchangedappropriately.

As explained above, the semiconductor device according to the inventionincludes a layer with low electrical resistance; a semiconductivesubstrate region on the layer with low electrical resistance; one ormore electrodes on the surface of the semiconductive substrate region;and an electrode on the back surface of the layer with low electricalresistance; the semiconductive substrate region providing a current pathwhen the semiconductor device is ON and being depleted when thesemiconductor device is OFF. The semiconductive substrate regionincluding regions of a first conductivity type and regions of a secondconductivity type wherein the regions of the first conductivity type andthe regions of the second conductivity type are extended in parallel toeach other vertically and arranged alternately with each otherhorizontally. Each of the regions of the first conductivity type includea plurality of second buried regions of the first conductivity typealigned vertically at a predetermined pitch. Each of the regions of thesecond conductivity type include a plurality of first buried regions ofthe second conductivity type aligned vertically at the predeterminedpitch.

The semiconductor devices of the invention are manufactured by themethod that includes the steps of epitaxially growing a highly resistivelayer laminate on the layer with low resistance, and diffusingimpurities into the highly resistive layer laminate to form the firstburied regions and the second buried regions. The semiconductor deviceand its manufacturing method according to the invention exhibit thefollowing effects.

By using techniques such as epitaxial growth and impurity diffusion, acharacteristic alternating conductivity type layer has been realizedwithout forming trenches with a large aspect ratio and filling eachtrench with a high-quality epitaxial layer.

The resulting easy depletion of the first buried regions and the secondburied regions and the resulting increased impurity concentrations inthe first buried regions and the second buried regions facilitatethinning the semiconductive substrate region that includes thealternating conductivity type layer, further resulting in loweredon-resistance, e.g., lowered by 80% from the conventional value, and animproved tradeoff relation between the on-resistance and the breakdownvoltage.

By applying the present invention to power devices, novel semiconductorpower devices which facilitate dramatically reducing the electricalpower loss are realized.

Although the present invention has been described in connection withspecific exemplary embodiments, it should be understood that variouschanges, substitutions and alterations can be made to the disclosedembodiments without departing from the spirit and scope of the inventionas set forth in the appended claims.

1. A semiconductor device comprising: a layer with low electricalresistance; a semiconductive substrate region on the layer with lowelectrical resistance; the semiconductive substrate region providing acurrent path when the semiconductor device is ON and being depleted whenthe semiconductor device is OFF; the semiconductive substrate regioncomprising regions of a first conductivity type and regions of a secondconductivity type; the regions of the first conductivity type and theregions of the second conductivity type being arranged alternately witheach other; a first major surface above the semiconductive substrateregion; a second major surface on the back surface of the layer with lowelectrical resistance; one or more trenches formed in depth from thefirst major surface to at least portions of the regions of a firstconductivity type of the semiconductive substrate region; one or moreinsulating films are formed inside of the trench; and one or moreinsulators or one or more electrodes are buried in the trenches enoughto cover over the entrances of the trenches.
 2. The semiconductor deviceaccording to claim 1, wherein a portion of a side of the trench isalmost perpendicular to the first major surface.
 3. The semiconductordevice according to claim 1, wherein a most upper portion of theinsulator or the electrode buried in the trench is formed by aninsulator.
 4. The semiconductor device according to claim 1, wherein theregions of the first conductivity type and the regions of the secondconductivity type are shaped with stripes in a cross sectional viewbeing in parallel with the first major surface.